Reservoir Engineering 2 Course (1st Ed.)

1. About This Course

2. Syllabus

3. Resources

4. Training Outline (beta)

1. Coning Introduction

2. Coning types

3. Coning dependency

Coning

Coning is a term used to describe the mechanism underlying the upward movement of water and/or the down movement of gas into the perforations of a producing well.

Coning can seriously impact the well productivity and influence the degree of depletion and the overall recovery efficiency of the oil reservoirs.

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problems of water and gas coning

The specific problems of water and gas coning are listed below.Costly added water and gas handling

Gas production from the original or secondary gas cap reduces pressure without obtaining the displacement effects associated with gas drive

Reduced efficiency of the depletion mechanism

The water is often corrosive and its disposal costly

The afflicted well may be abandoned early

Loss of the total field overall recovery

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Coning Sequences

Delaying the encroachment and production of gas and water are essentially the controlling factors in maximizing the field’s ultimate oil recovery.

Coning can have an important influence on operations, recovery, and economics.

Coning is primarily the result of movement of reservoir fluids in the direction of least resistance, balanced by a tendency of the fluids to maintain gravity equilibrium.

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Original reservoir static condition

Let the original condition of reservoir fluids exist as shown schematically in Figure, water underlying oil and gas overlying oil.

For the purposes of discussion, assume that a well is partially penetrating the formation so that the production interval is halfway between the fluid contacts.

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Original reservoir static condition (Cont.)Production from the well would create pressure

gradients that tend to lower the gas-oil contact and elevate the water-oil contact in the immediate vicinity of the well.

Counterbalancing these flow gradients is the tendency of the gas to remain above the oil zone because of its lower density and of the water to remain below the oil zone because of its higher density.

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Gas and water coning

These counterbalancing forces tend to deform the gas-oil and water-oil contacts into a bell shape as shown schematically in the Figure.

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Forces affecting fluid flow distribution around the well boresThere are essentially three forces that may affect

fluid flow distributions around the well bores. These are:Capillary forces

Capillary forces usually have a negligible effect on coning and will be neglected.

Gravity forcesGravity forces are directed in the vertical direction and arise

from fluid density differences.

Viscous forcesThe term viscous forces refers to the pressure gradients’

associated fluid flow through the reservoir as described by Darcy’s Law.

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Coning occurrence

Therefore, at any given time, there is a balance between gravitational and viscous forces at points on and away from the well completion interval.

When the dynamic (viscous) forces at the wellbore exceed gravitational forces, a “cone” will ultimately break into the well.

We can expand on the above basic visualization of coning by introducing the concepts of:Stable coneUnstable coneCritical production rate

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Stable vs. unstable cone

If a well is produced at a constant rate and the pressure gradients in the drainage system have become constant, a steady-state condition is reached.

If at this condition the dynamic (viscous) forces at the well are less than the gravity forces, then the water or gas cone that has formed will not extend to the well.

Moreover, the cone will neither advance nor recede, thus establishing what is known as a stable cone.

Conversely, if the pressure in the system is an unsteady-state condition, then an unstable cone will continue to advance until steady-state conditions prevail.

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Unstable and pseudo-stable cone

If the flowing pressure drop at the well is sufficient to overcome the gravity forces, the unstable cone will grow and ultimately break into the well.

It is important to note that in a realistic sense, stable system cones may only be “pseudo-stable” because the drainage system and

pressure distributions generally change.

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pseudo-stable cone examples

For example, with reservoir depletion, the water-oil contact may advance toward the completion interval, thereby increasing chances for coning.

As another example, reduced productivity due to well damage requires a corresponding increase in the flowing pressure drop to maintain a given production rate. This increase in pressure drop may force an otherwise

stable cone into a well.

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critical production rate

The critical production rate is the rate above which the flowing pressure gradient at the well causes water (or gas) to cone into the well.

It is, therefore, the maximum rate of oil production without concurrent production of the displacing phase by coning.

At the critical rate, the built up cone is stable but is at a position of incipient breakthrough.

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maximum water-free and/or gas-free oil production rateDefining the conditions for achieving the maximum

water-free and/or gas-free oil production rate is a difficult problem to solve.

Engineers are frequently faced with the following specific problems:Predicting the maximum flow rate that can be assigned

to a completed well without the simultaneous production of water and/or free-gas

Defining the optimum length and position of the interval to be perforated in a well in order to obtain the maximum water and gas-free production rate

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Cone creation

Calhoun (1960) pointed out that the rate at which the fluids can come to an equilibrium level in the rock may be so slow, due to the low permeability or to capillary properties,

that the gradient toward the wellbore overcomes it.

Under these circumstances, the water is lifted into the wellbore and the gas flows downward, creating a cone.

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Rapidity of the coning

Not only is the direction of gradients reversed with gas and oil cones, but the rapidity with which the two levels will balance will differ.

Also, the rapidity with which any fluid will move is inversely proportional to its viscosity, and, therefore,

the gas has a greater tendency to cone than does water.

For this reason, the amount of coning will depend upon the viscosity of the oil compared to that of water.

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Coning dependency

It is evident that the degree or rapidity of coning will depend upon the rate at which fluid is withdrawn from the well

and upon the permeability in the vertical direction kv compared to that in the horizontal direction kh.

It will also depend upon the distance from the wellbore withdrawal point

to the gas-oil or oil-water discontinuity.

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The elimination of coning

The elimination of coning could be aided by shallower penetration of wells where there is a water

zone or

by the development of better horizontal permeability. Although the vertical permeability could not be lessened, the

ratio of horizontal to vertical flow can be increased by such techniques as acidizing or pressure parting the formation. • The application of such techniques needs to be controlled so that

the effect occurs above the water zone or below the gas zone, whichever is the desirable case.

This permits a more uniform rise of a water table.

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Coning restabilization

Once either gas coning or water coning has occurred, it is possible to shut in the well and permit the contacts to restabilize.

Unless conditions for rapid attainment of gravity equilibrium are present, restabilization will not be extremely satisfactory.

Fortunately, bottom water is found often where favorable conditions for gravity separation do exist.

Gas coning is more difficult to avoid because gas saturation, once formed, is difficult to eliminate.

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Solving the coning problem

There are essentially three categories of correlation that are used to solve the coning problem. These categories are:Critical rate calculations

Breakthrough time predictions

Well performance calculations after breakthrough

The above categories of calculations are applicable in evaluating the coning problem in vertical and horizontal wells.

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1. Ahmed, T. (2010). Reservoir engineering handbook (Gulf Professional Publishing). Chapter 9

1. Conning Vertical Well:A. Critical Rate Correlations

B. Breakthrough Time

C. Breakthrough Performance